Multiple lead voltage probe and method of making same

ABSTRACT

A probe head includes analog amplifier inputs, a ground plane, and hundreds of probe leads between the inputs and the pins of a circuit under test. The customer defines the grounded pins of the circuit under test. Non-active probe leads, i.e. leads corresponding to the grounded pins are connected to the ground plane, maximizing the connections between the grounds of the probe and the circuit under test and minimizing unequal ground potentials. The probe circuit is on a probe circuit board, while the connections between the ground plane and the leads are fusible elements on a separate ground personality board. The probe is placed on a simulated circuit under test, the grounded pins on the circuit under test are protected by an insulating cap, and a voltage is placed on the remainder of the pins to fuse the elements corresponding the active probe leads.

CROSS REFERENCE TO RELATED APPLICATION(S)

This is a divisional of copending application Ser. No. 08/518,408 filedon Aug. 23, 1995, now U.S. Pat. No. 5,654,647 , which is acontinuation-in-part of U.S. patent application Ser. No. 08/384,296,filed Feb. 3, 1995, now U.S. Pat. No 5,625,299.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention in general relates to voltage probes for passing a testsignal from an electronic circuit element to be tested to anoscilloscope or other electronic measurement device, and moreparticularly to such a probe that has multiple leads densely packed intoa small area, certain of which leads are grounded. The invention alsorelates to a method of making such a probe with grounded leads.

2. Description of the Related Art

Voltage probes are commonly used to pass analog test signals from acircuit under test to an oscilloscope or other electrical or electronictest instrument. Such an electronic probe must be capable of passing anelectrical signal on a node or pin of the circuit under test to the testinstrument without distorting it, i.e. with high signal integrity.Further, it should not apply any voltage or current to the circuit undertest. Present-day electronic circuits operate over frequencies from DCto several gigahertz. Thus, test probes capable of being used with awide variety of circuits must be able to provide high signal integrityover a wide band width of frequencies.

Integrated and hybrid circuits are becoming both more complex andsmaller, leading to ever higher numbers of package leads crowded intoless and less space, that is, the leads are becoming extremely densewith very tight pitches. The parent application referred to above hasessentially the same high density of circuitry as state-of-the-artintegrated and hybrid circuits. The closeness of the individual leads insuch high density circuits generally results in coupling between theleads and associated noise, distortion, etc. which is unacceptable involtage probes. Thus, a probe that has a high density of leads yet inwhich the noise and coupling is virtually eliminated.

As will become evident below, the present invention solves the aboveproblem in part by grounding all the leads of the probe that areconnected to the pins of the circuit under test that are grounded. Thisusually results in a large number of grounded leads. Since the leadsthat are grounded are determined by the application of the probe, thatis the particular chip to which a probe is applied this also results ina probe that is, in part, custom manufactured for its end application.It would be highly desirable to have a simple yet effective method ofmanufacturing probes with such customized grounded leads.

Voltage probes are sophisticated electronic instruments and thus are notinexpensive. Thus, it would also be highly desirable to have a voltageprobe, the leads of which could be customizably grounded for itsspecific application, and, at the same time, could be still be used inmany applications with little additional expense.

SUMMARY OF THE INVENTION

The present invention solves the problem of noise and coupling betweenclosely packed leads by providing a probe designed so that its groundand the ground of the circuit under test will be essentially at the sameground potential. This is done by maximizing the number of probe leadsthat are connected both to the probe ground and the ground of thecircuit to be tested. This is facilitated by allowing the user of theprobe to define the pins on the circuit to be tested which are to begrounds, and then designing the probe so that all probe leads thatcorrespond to grounded pins on the circuit under test are connected tothe probe ground. This all but eliminates noise and coupling.

The invention solves the problem of having a probe that is customizableand yet can be used in many applications with little expense byproviding a voltage probe with a ground personality board. The groundpersonality board includes a circuit that connects selected leads toground. By simply replacing the ground personality board, the probe canbe used in many different applications.

The invention also provides a method of easily customizing the leadsthat are connected to ground. In the method, a board in which each leadis connected to ground through a fuse is installed in the probe. Then avoltage that is high enough to destroy the fuse, yet low enough so asnot to harm any portion of the probe circuit, is applied to each leadthat is selected to be an active lead. This voltage blows the fuse toground for that lead. The board thus becomes a ground personality boardin which only the leads that are selected to be grounds for the specificapplication are grounded. Preferably the fuses are wire bonds connectingeach lead to ground.

The invention provides an analog voltage probe comprising: a probecircuit board having an analog probe circuit including a plurality ofprobe input leads and a probe ground; a ground personality circuit boardhaving a ground personality circuit comprising: a plurality ofelectrical conductors and a ground element, each of the conductorsincluding a terminal adapted to be electrically connected to a circuitunder test, and selected ones of the conductors electrically connectedto the ground element; and an electrical connector connecting each ofthe electrical conductors to one of the probe input leads and the groundelement to the probe ground. Preferably, each of the terminals isadapted to be connected to one of the pins of the circuit under test,and the selected ones of the conductors are those corresponding to thepins of the circuit under test that are connected to the device ground.Preferably, the electrical connector comprises an elastomer elementcompressible between the probe circuit board and the ground personalitycircuit board. Preferably, the ground personality circuit board includesa fusible element connecting the selected ones of the conductors to theground element.

In another aspect the invention provides an analog voltage probecomprising: an analog amplifier; a plurality of terminals each adaptedto be connected to one of the pins of the circuit under test; aplurality of probe input leads, each of the leads connected to one ofthe terminals and connectable to the analog amplifier; a probe ground;and a fusible element connecting each of the probe input leads to theprobe ground. Preferably, the fusible element comprises an integratedcircuit bond wire. Preferably, the bond wire fuses at between 0.5amperes and one ampere current. Preferably, the bond wire comprises ametal selected from the group consisting of gold, aluminum, andplatinum. Preferably, the analog amplifier is on a probe circuit boardand the fusible elements are on a ground personality circuit boardseparate from the probe circuit board.

In a further aspect the invention provides a method of making an analogvoltage probe, the method comprising: providing a probe circuitcomprising: an analog amplifier; a plurality of terminals each adaptedto be connected to one of the pins of the circuit under test; aplurality of probe input leads, each of the leads connected to one ofthe terminals and connectable to the analog amplifier; a probe ground;and a fusible element connecting each of the probe input leads to theprobe ground; and fusing selected ones of the fusible elementscorresponding to the active pins of the circuit under test. Preferably,the step of fusing comprises connecting the probe circuit to a simulatedcircuit under test having a plurality of pins, and applying a voltage toselected ones of the pins. Preferably, the step of fusing comprises:connecting the probe circuit to a device having a plurality of pinscorresponding to the pins of the device under test, each pinelectrically connected to one of the terminals; placing an insulatingcap on the ones of the pins that are designated as grounds in thecircuit under test; and applying a voltage to the remainder of the pins.Preferably, the voltage is between 3 volts and 5 volts.

In yet another aspect, the invention provides a method of making avoltage probe of the type connectable to a circuit to be tested, thecircuit to be tested having a circuit ground and a plurality of circuitnodes, the method comprising the steps of: providing an analog voltageprobe comprising a probe circuit board including a probe ground and aplurality of input leads, each of the input leads comprising a trace onthe circuit board, and each of the plurality of input leads adapted tobe connected to a specific one of the circuit nodes; and connecting eachof the input leads corresponding to selected ones of the circuit nodesto the probe ground. Preferably, the step of connecting comprisesselecting which of the circuit nodes are connected to circuit ground andconnecting to probe ground each of the input leads corresponding to thenodes selected to be connected to circuit ground. Preferably, the stepof connecting the input leads to the probe ground comprises: providing aground personality board including a plurality of conductors, a groundelement, selected ones of the plurality of conductors being connected tothe ground element; and electrically connecting the ground personalityboard to the probe circuit board with each of the input leadscorresponding to selected ones of the circuit nodes being connected toone of the selected conductors, and the ground element being connectedto the probe ground. Alternatively, the step of connecting the inputleads to the probe ground comprises soldering. Preferably, the step ofselecting is performed by the user of the circuit. Preferably, the stepof connecting comprises maximizing the number of the selected circuitnodes and therefore maximizing the number of connections between thecircuit ground and the probe ground. Preferably, the step of providingfurther includes providing a plurality of intermediate leads, eachintermediate lead located between an adjacent pair of the input leads,and the step of connecting further comprises connecting each of theintermediate leads to the probe ground, whereby each of the input leadsis separated from adjacent input leads by a grounded intermediate lead.

The invention not only provides a hand held voltage probe that includeshundreds of probe channels while maintaining high band width and highsignal integrity, it also does this simply, thereby allowing the probeto be manufactured relatively inexpensively. Numerous other features,objects and advantages of the invention will become apparent from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block circuit diagram of the preferred embodiment of anelectronic probe system according to the invention;

FIG. 2 is block circuit diagram of a typical high density plastic quadflat pack (PQFP) probe head of the probe system of FIG. 1;

FIG. 3 is a detail of the circuit board containing the probe headcircuit of FIG. 2, showing the structure and arrangement of the circuitincluding leads and integrated circuit chips;

FIG. 4 is a cross section through the line 4--4 of FIG. 3;

FIG. 5 is a perspective view of a probe head according to the inventionand a PQFP showing how the probe head attaches to the PQFP;

FIG. 6 shows a circuit diagram of the input circuit, including the inputdivider network, of the integrated circuit of FIG. 7;

FIG. 7 is a block circuit diagram of an integrated circuit chip of FIG.3;

FIG. 8 is an exploded view of a portion of the probe head of analternative preferred embodiment of the invention showing the circuitboard, the ground personality board, and the connections between theseboards;

FIG. 9 shows in detail an exemplary portion of one side of the groundpersonality board of FIG. 8;

FIG. 10 shows in detail an exemplary portion of the other side of theground personality board of FIG. 8; and

FIG. 11 is a plane view of a calibration board and illustrates themethod of blowing the wire-bond fuses; and

FIG. 12 is a cross-section of the calibration board of FIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENT

1. Overview

FIG. 1 shows the preferred embodiment of an analog voltage probe system100 in which the invention is implemented. It should be understood thatthe specific system shown in the figures and described herein, isexemplary. That is, it is intended to show preferred examples of theinvention so that those skilled in the art can fully understand andimplement it. It is not intended to limit the invention to the specificexamples described and shown herein.

In this disclosure, the term "electrically connected" when applied totwo electrical elements, such as an input and an output, means that anelectrical signal, such as a voltage, a current, an analog signal, or adigital signal, will pass from one element to the other. This is indistinction to a physical connection by electrical components. Forexample, an input and an output may be physically connected by wires,amplifiers, transistors, resistors and other electrical components, butno signal will pass from the input to the output because one or more ofthe switching or amplification components may be off. In this case, theinput and output are not "electrically connected". In this disclosure"amplifier" means an electronic circuit that passes signals, usuallychanging the amplitude, without significant distortion, and includes 1:1amplifiers as well as negative amplifiers, not just amplifiers with apositive gain.

Probe system 100 includes three plastic quad flat pack (PQFP) probes,such as 101, each of which include a probe head, such as 103, and two ofcoaxial cables 115. Each of probe heads 102, 103 and 104 has a specificnumber of inputs 105 and is mechanically enclosed in a probe body 515(FIG. 5), which probe body is designed to be easily held in one hand andeasily mechanically coupled to a specific plastic quad flat pack (PQFP)510 (FIG. 5). The circuitry of the probe head, such as 103, is locatedon a circuit board 306 (FIGS. 5 and 6) in the probe body 515. The probeinputs 105 in the form of a pod array 520 are designed to be easilyelectrically coupled to the inputs of the specified PQFP. For example,probe head 102 is designed to couple to a PQFP with 240 pins, probe head103 is designed to couple to PQFP with 208 pins 530, while probe head104 is designed to couple to a PQFP having 160 pins.

The exemplary probe system 100 also includes a general purpose singlepoint probe 106, which includes nine probe tips 108 and a circuit pod109. Each probe tip 108 is connected to pod 109 via a 50 ohm coaxialcable 110. General purpose probe 106 may be used to probe circuits forwhich no specific probe head is available.

The probe system 100 has two outputs 129 and 130. Likewise most of thesystem components, such as probe heads 102-104 and pod 109 have twooutputs, such as 111 and 112. In each case we shall refer to one outputas the "A" output and the other as the "B" output. Each probe head 102,103, 104 can connect any of its inputs 105 to either or both of itsoutputs. For example, probe head 103 can connect any of its 240 inputsto either or both of its outputs, 111 and 112. Each of the separatepaths through the system that a signal can follow from a selected one ofthe inputs 105 or probe tips 108 to a selected one of the probe outputs129 and 130, defines a channel. In the case of the probe 100 and itscomponents in general, half the channels pass through the "A" output 129and half through the "B" output 130. As a shortened notation, in someinstances below we shall refer to electronic circuits or elements on the"A" output side of the system, or to a component as an "A" or "B"channel component.

Probe system 100 also includes a printed circuit board (PCB) 120 whichfits into a logic analyzer 133 designed to interface with the probe,which logic analyzer is sometimes referred to in the art as a "mainframe". PCB 120 contains a channel selection programmer circuit 121, acalibration control circuit 122, an offset control circuit 123, and aprobe power circuit 124, which circuits share a microprocessor 125 andits associated memory 126; e.g. the channel selection programmer 121includes memory 126 and microprocessor 125 in that channel selectionsoftware stored in memory 126 is used by microprocessor 125 to provideoutput signals that cause the programmer 121 to output data via cable160 to program latches (not shown) in the probe 100. Microprocessor 125and memory 126 are not on PCB 120 but are in main frame 133, and thusare shown with a dotted line around them. The various circuits 121through 126 on PCB 120 include other electrical elements andinterconnections that, to those skilled in the art, will be clear fromthe following description.

PCB 120 also includes a second level multiplexer 127. Multiplexer 127 isimplemented as an integrated circuit (IC) die on PCB 120, and is capableof connecting any of its 8 inputs to either or both of its outputs, 129and 130. In addition, probe system 100 includes means 140 for inputtingcontrol signals, such as for programming second level multiplexer 127,probe heads 102-104, and pod 109. In the preferred embodiment, means 140includes dials 141 and a keyboard 142, though almost any mechanism forgenerating electrical control signals may be used. In the preferredembodiment, the dials 141 are located on the front of the logic analyzer133, and the keyboard is a computer work station keyboard; however, forsimplicity they are shown on a common control signal input means 140. Inthe preferred embodiment several dials 145 together with channelselection programmer 121 comprise selection means 143 for selecting oneof the probe inputs 105, 108 and one of the probe outputs 129 or 130,while one dial 146 together with channel selection programmer 121comprise gain selection means 144 for selecting one of a plurality ofpossible gains for signals passing from said selected input to saidoutput.

The outputs 111, 112, of probe heads 102-104 and pod 109 are connectedto second level multiplexer 127 via standard 50 ohm coaxial cables 115.The outputs 129, 130 of second level multiplexer 127 are connectable toa test instrument, such as an oscilloscope 150, via 50 ohm coaxialcables 149. Control PCB 120 is connected to probe heads 102-104, pod 109and second level multiplexer 127 via a multiwire cable 160. Multiwirecable 160 includes conventional power lines, a serial interfaceincluding data and clock lines, and other lines. In the preferredembodiment, coaxial cables 115 and wires 160 are bound together in asingle cable.

In one embodiment, invention also includes a ground personality board810 (FIG. 8) that is sandwiched between the circuit board 806 (FIG. 8)and the coupling to the pins 530 of the PQFP. The ground personalityboard allows the probe to be customized to a particular PQFP design.

As can be seen from the above description of the probe system, itincludes hundreds of channels that are packed into a device, such asprobe head 103, that can be held in one hand. Obviously, the channelsmust be physically very close to one another. FIG. 3 shows a portion ofa probe head that is about one-half inch along the horizontal dimension.This portion includes about 50 leads 302 that connect to integratedcircuit chips, such as 202, all of which are on a printed circuit board306. The invention involves the physical design and structure of theleads 302, circuit board 306, probe head circuit 303, and integratedcircuit chips which permits so many channels to be packed so denselywhile at the same time retaining the high signal integrity and bandwidth required for instrumentation purposes and permitting thecustomization probe to be easily changed for use with different PQFP'sor other IC devices.

2. Detailed Description

Turning to FIG. 2, a semi-block circuit diagram of two-hundred-and-eightpin PQFP probe 101 is shown. Probe 101 includes probe head 103 andcoaxial cables 115. Probe head 103 includes memory 201, four customintegrated circuit chips 202-205, two-hundred-and-eight probe inputs105, two-hundred-and-eight input resistors, such as 209, eight outputresistors, such as 210, 50 ohm microstrip "coax" 214 and 215, andmicrostrip terminating resistors 212 and 213.

Memory 201 is connected to calibration control circuit 122 (FIG. 1)through wire cable 240, preferably a serial interface connection, incable bundle 160. Integrated circuit chip 202 is connected to channelselection programmer 121 via cable 242 in cable bundle 160, alsopreferably a serial interface connection. Each of chips 202 through 205is serially connected to the next chip via a circuit connection such as243. This connection via line 242 and integrated circuit connectionssuch as 243 transfers data sequentially through latches (not shown) inthe IC's to program which of the probe channels will be active, that is,which probe channels will pass a signal to test instrument 150. Each ofprobe inputs 105 connect to one of IC input pins 230 through a resistor209. Each of IC chips 202-205 include an "A" output and a "B" output.Each of the "A" outputs connect to probe head output 111 through aresistor 210 and via stripline 214. Each of the "B" outputs connect toprobe head output 112 through a resistor 211 and via stripline 215. Eachstripline 214, 215 is connected to ground via terminating resistors 212,213 respectively. Note that in this disclosure ground is indicated by atriangle, such as 220.

Resistors 209 are each preferably 150 ohm resistors imbedded in theprinted circuit board 306 (FIG. 3) of the probe head 103. Resistors 210,211, 212, and 213, are preferably 50 ohms. Each of IC chips 202-205 isidentical and is a custom integrated circuit chip as will be describedbelow.

Turning to FIG. 3, a portion of a probe head 103 and probe head circuit303 is shown. This portion of the probe head 103 has been greatlyexpanded to show the detail; the actual size of the portion shown isapproximately three-eighths by one-half inches, though the various partsmay not be to scale. The portion of probe head circuit 303 in FIG. 3includes one integrated circuit chip 202 and a portion of another, leads302, and a ground element 304. Leads 302 are connected to the input pins230 of chips 202, preferably by wire bonds 305. Leads 302 include inputleads, such as 308, which connect the chip 202 with inputs 105, andintermediate leads 309 which connect chip 202 with ground element 304.Each input lead 308 includes a terminal 310, a buried resistor 209, anda trace portion 312. The terminal portion 316 is connected to pod array520 (FIG. 5) by means of a plated through via 316. The terminals 310 arein a pair of staggered rows 321 and 322. Ground element 304 is connectedto bottom ground plane 412 (FIG. 4) by way of plated through vias, suchas 326. There are sufficient such vias 326 so that ground element 304can be considered to be an extension of ground plane 412. Insulatinggaps, such as 328, separate input leads 308 and intermediate leads 309.For selected ones, such as 338, of input leads 308, the gaps 328 arespanned by conductors, such as 332, which connect these certain inputleads 338 to the ground element 304. We shall see below how these inputleads 338 are selected.

A cross-section of probe head 103 through line 4--4 in FIG. 3 is shownin FIG. 4. The cross-section shows circuit board 306 and traces 312formed on the board. Circuit board 306 includes top ground plane 410,bottom ground plane 412 and various other conducting elements, such as430 and 434, which are separated by insulating layers 420, 421, 422, and427. Conductors 214 and 215 carry the output signals. Conductors 434 arelines such as 240 and 242 (FIG. 2) which carry data signals to the chips202, power supply lines, etc.

Preferably, traces 312 are made of gold-plated copper or other suitabletrace materials, are between 1 and 4 mils wide, i.e. the horizontaldirection in FIG. 4, and between 0.5 and 3 mils thick, i.e. the verticaldirection in FIG. 4. Most preferably the traces 312 are between 1 and 3mils wide and about 1 mils in thickness.

Preferably dielectric layer 420 between traces 312 and ground plane 410is thin. It has been found that there is a tradeoff here: the thinnerthe dielectric layer 420, the lower the coupling between the tracesbecomes, but the higher the input capacitance becomes. Preferably thethickness is between 4 mils and 6 mils. Most preferably it is 5 milsthick. The dielectric constant of material 420 is low, preferablybetween 0.5 and 5. Most preferably it is 2.2. Dielectric 420 ispreferably polytetrafluoroethylene, and most preferably thepolytetrafluoroethylene sold under the trademark DICLAD 880 by ArlonMicrowave Materials Division, although other materials with the aboveproperties may also be used.

Buried resistors 209 are each preferably 13 mils wide by 20 mils long,made of 100 ohm per square material 5 microinches thick, and arepreferably 150 ohms. The use of buried resistors permits the inputdamping resistance to be located very close to the input, whichsignificantly improves damping of the circuit response. At the same timeit permits a high density of resistors. Insulating layer 427 is actuallyformed in several layers, but the details of these layers are not shownsince these details are either redundant of details already discussed orrelate to conventional materials and thicknesses. Insulating layers 421and 422 are preferably made of FR4, which is well-known in the PC boardart, or other conventional PC board material.

The combination of thin traces 312 and a thin, low dielectric constantseparation between traces 312 and ground plane 410 is critical forproviding a high signal integrity and wide band width probe with a highdensity of channels. The combination of thin traces 302 and thindielectric material 420 is important for reducing coupling between leadsthrough the ground. The low dielectric constant results in lowcapacitance between the leads 302 and ground 410.

An important feature of the circuit on circuit board 306 is that everyother lead 302 is an intermediate lead connected to ground. Thus, eachinput lead is separated from every other input lead by a ground. Thisfeature greatly reduces coupling and is also critical for providing highsignal integrity and wide band width.

FIG. 5 shows a PQFP probe 101 and a PQFP 510. Circuit board 306 isenclosed in probe body 515 and, as indicated above, makes contact withthe pod array 520 by means of plated vias. Pod array 520 forms theinputs 105 to probe 101. It is of a special design which is disclosed ina separate patent disclosure. It is designed to make good electriccontact between the nodes of the circuit under test, i.e. the pins 530,538 of PQFP 510, and the terminals 310 of input leads 308.

An important feature of the probe 101 is that it is designed so that itsground 304 and the ground 504 of the circuit 510 under test can be madeas close as possible. That is, the ground potential of the ground 304and the ground 510 are essentially the same. The ground 504 of PQFP 510is shown only generally and in ghost in FIG. 5 since it will usually bean element similar to element 304 of FIG. 3 that is located within theflat pack structure. The two grounds 304 and 504 are made as close aspossible by maximizing the number of probe leads 308 that are connectedto both the probe ground element 304 and the ground 504 of the circuit510 to be tested. This is facilitated by allowing the user of the probeto define the pins 538 on the circuit 510 which are to be grounded, andthen designing the probe so that all probe input leads 308 thatcorrespond to grounded pins 538 on the circuit 510 under test areconnected to the probe ground element 304. That is, the user, whenordering a probe system 100, will specify certain desired features, suchas the number of probe heads 102, 103, etc., the number of inputs 105desired for each probe head, and the particular circuit package that theprobe head is to mate with, such as a PQFP. According to the invention,the user will also designate which selected pins 538 of the package 510are grounds. Probe heads 102, 103, etc. are then manufactured asdescribed above, or premanufactured probe heads are taken frominventory. Selected ones, such as 338, of the input leads 308 correspondto a grounded pin 538; that is, selected leads 338 connect to a groundedpin 538 by means of terminals, such as 339, vias, such as 340, and podarray 520. Before shipping to the customer, electrical connections aremade between each of the input leads 338 which correspond to a groundedpin 538, and the ground element 304. Preferably this electricalconnection is made by simply applying a drop of solder between theterminal 339 of the lead 338 and the ground element 304, as shown at332. Thus, the ground plane 412 of the probe head 103 is then connectedto the ground of the circuit 510 under test. The more of the leads 308that are connected to grounded pins 538 and to ground element 304, theless the total impedance between the grounds of the circuit 510 and theprobe 103, and the closer the potential of the grounds will be. By themanufacturing method of the invention just described, the number ofinput leads that connect the grounds of the probe head 103 and thecircuit 510 is maximized. Thus, signal distortions and other potentialproblems caused by fewer grounds are minimized.

From the above it is seen that in the usual applications of theinvention both the intermediate leads 308 and selected ones 338 of theinput leads will be connected to ground element 304. Those ones 311 ofthe input leads 308 that are not connected to ground, but rather areconnected to pins 535 of circuit under test that are active, i.e. carrya signal at some point in the cycle of the circuit 510, are referred toherein as "active leads" 311.

FIGS. 8 through 10 show an alternative and preferred embodiment of aprobe 101 in which the leads that are not active are grounded. In thisembodiment, instead of the individual leads 338 on the circuit board 306that are selected to be grounded being grounded by a solder ball 332, aseparate board 810 is provided that includes a circuit (FIGS. 9 and 10)that connects each of the selected leads to ground.

FIG. 8 shows an exploded view of a portion 803 of a probe head showing acircuit board 806, the ground personality board 810, a firstboard-to-board cassette 812 which locates the electrical connectors 822which connect the circuit board 806 to ground personality board 810, anda second cassette 814 which locates the electrical connectors 824between the ground personality board 810 and the pins 530 (FIG. 5) of acircuit under test 510 or the pins 1130 a simulated circuit under test1110 (FIG. 11) on the calibration board 1105. For simplicity, thedetails of the circuits on circuit board 806 and ground personalityboard 810 are not shown in FIG. 8.

Circuit board 806 is identical to circuit board 306 except that it doesnot contain the solder connections 332. Rather than provide an extrafigure that reproduces FIG. 3 without the solder balls, we shall discussthe circuit of board 806 by reference to FIG. 3 and refer to theelements of the circuit of circuit board 806 by the same numbers as theelements of the circuit of circuit 306. Cassette 812 includes fourrectangular slots 816. An elastomer electrical connector member 822,which is embedded with fine wires, fits into each of slots 816 toconnect the terminals 310 and ground plane 304 of circuit 806 to theterminals 910 (FIG. 9) and ground plane 904 on the first side 811 ofground personality board 810. Similarly, cassette 814 includes fourrectangular indentations 818 ito which fit four elastomer connectors 824which connect the terminals 1080 (FIG. 10) on the bottom or second side813 of ground personality board 810 with the pins 530 of a circuit undertest 510. Cassette 814 also includes sixteen oblong recesses 840 whichprovide space for the fuse wires 1020 so they are not crushed. Cassettes812 and 814 are made of an insulating material, such as a polycarbonate,other plastics, or other moldable insulating material. The details ofthe elastomer connectors 822, 824 and how they function are described inU.S. Pat. No. 5,314,342. Dowels, such as 820, fit in bores, such as830-833, in the circuit board 806, ground personality board 810, andcassettes 812, 814 to align the boards and cassettes. Each of thecassettes 812 and 814 includes a rim, such as 842, which is of adimension so that the edge 843 of ground personality board snap-fitsinto the rim 842. The rim is not shown on cassette 812 since it is onthe bottom side which is not visible in FIG. 8. Thread forming screws,such as 844, which screw into bores such as 845 and 846, fasten theassembly to the probe body 515, compressing the elastomer connectors822, 824 and making electrical contact between the circuit boards 810and 812.

FIG. 9 shows the top or first side of ground personality board. Theboard 810 includes a circuit 907 that includes ground plane 904,conductors 908, each of which includes a trace 909 and a terminal 910,and plated through vias 915, 916. Circuit 907 is designed to connect tothe circuit on circuit board 806. Conductors 908 are made of gold-platedcopper or other suitable trace materials. Ground plane 904 is made ofcopper. FIG. 10 shows the bottom or second side 813 of groundpersonality board 810. This side of the board includes a circuit 1007that includes conductors 1008, each of which includes a trace 1009 and aterminal 1080, ground planes 1004, and fuses 1020. As can be seen bycomparing the two sides 811, 813 of board 810, each of vias 915 connectone of terminals 910 to one of terminals 1080, and each of vias 916connect the ground plane 916 with the ground plane 1004. Electronically,each of ground planes 1004 is an extension of ground plane 904 becauseof their connection by way of vias 916.

Prior to calibration of the probe 101, each conductor 1008 is connectedto one of the ground planes 1004 by a thin fuse, which in the preferredembodiment is an integrated circuit bond wire. Preferably fuse wire 1020is made of gold, aluminum, platinum or other suitable bond wire that isof an appropriate thickness so that it melts at about 0.5 amperes to 1ampere current. Typically fuse wire 1020 is a 0.8 mil gold wire thatfuses at about 0.8 or 0.9 amperes. Platinum wires typically fuse atlower amperages, such as about 0.5 amperes, thus may be used when it isparticularly important to protect the elastomer connectors 822, 824.

Returning to FIG. 8 and considering FIGS. 3, 9, and 10, it is seen thateach of traces conductors 1008 is an extension of one of leads 308 oncircuit board 806, since each terminal 1080 is connected to one ofterminals 910, which in turn is connected to one of terminals 310 viacassette 812 and elastomer 822. Thus, prior to calibration, each ofleads 308 is connected to ground. In the calibration process, each offuse wires 1020 that is connected to a conductor 1008 that correspondsto an active lead 311 is fused, disconnecting that lead from ground.

Turning to FIGS. 11 and 12, a calibration board 1105 is shown. FIG. 11shows a plane view of the calibration board 1105, while FIG. 12 shows asectional view through the lower row of pins 1140. It should beunderstood that only the portions of calibration board 1105 that arerelevant to the present invention are shown, and that, in general,calibration board 1105 will include many other features. Calibrationboard 1105 includes a simulated or dummy circuit under test 1110,terminal pins 1140, traces, 1120, a ground plane 1104, and a powersource 1150. A probe 101 is shown in ghost in FIG. 11 in the position itis in when it is calibrated. Dummy circuit under test 1110 has the sameshape and number of pins 1130 as the actual circuit under test to whichthe probe 101 is to be applied. Each circuit under test pin 1130 isconnected to a terminal pin 1140 via a trace 1120. The traces 1120 fanout from the simulated circuit under test so that the terminal pins 1140are sufficiently separated to be easily addressed manually, and so thateach pin 1140 can be of sufficient size and strength to withstand manythousands of calibration operations. Ground traces, such as 1122,connect ground plane 1104 with a contact 1124 on probe 101 that connectsvia contacts 871 and 872 (FIGS. 8 and 9) to the ground plane 1004 of theground personality board 810. A ground connector 1151 connects powersource 1150 to the calibration board ground plane 1104. Power source1150 has an output of between 3 and 5 volts, and preferably about 4volts, and from 0.5 to 2.0 amperes. A connector tip 1152 is connected topower source 1150 via a flexible connector 1154. Insulating caps 1160cover the ones of calibration board terminal pins 1148 that areconnected to simulated circuit under test pins 1138 that are to begrounded.

A probe 101 (FIGS. 5 and 11) is calibrated by first determining which ofpins 1130 are to be grounded, and then putting an insulating protectivecap 1160 over each of the calibration board terminal pins 1148 that areconnected to these pins 1138. The probe 101, shown in ghost in FIG. 11,to be calibrated is pressed down on simulated circuit under test 1110 sothat each of pins 1130 connect to one of terminals 1080 on board 810 andcontact 1122 electrically connects to ground plane 1004 via contacts 871and 872 (FIGS. 8 and 9). The terminal pins 1142 that are not capped, arethen contacted by connector tip 1152 which applies an approximately 4volt, 1 amp spike of power across the bond wire fuse 1020 (FIG. 10) thatconnects to the one of conductors 1008 that connects that pin to ground1004. The circuit is closed through contact 1124, ground trace 1122,ground plane 1104, and ground connector 1151. The current spike fusesthe wire 1020, disconnecting that terminal and its corresponding pinfrom ground, and thus permitting the corresponding lead 308 (FIG. 3) tobe an active lead 311. This completes the manufacturing process for theparticular ground personality board 810.

It is a feature of the invention that a different ground personalityboard 810 may be manufactured for each different circuit under test 510with which the probe 101 is intended to be used. When the probe 101 isto be used with a particular circuit under test 510, the correspondingground personality board 810 is inserted in the probe body 515. Thus, ata relatively small cost, a probe 101 can be customized for use with aparticular circuit under test 510.

FIG. 7 is a block circuit diagram of an integrated circuit chip 202.Chip 202 includes an "A" channel muxamp 704 and programmable outputstage 705 and a "B" channel muxamp 706 and programmable output stage707. The "A" and "B" channels are identical and thus only one will bedescribed. Muxamp "A" 704 is a 54:1 muxamp that includes three 18:1muxamps 710, 711, and 712. Again, each of these three muxamps areidentical, and thus only muxamp 710 will be discussed in detail. Muxamp710 can be thought of as an 18:1 multiplexer 720, a feedbackdifferential amplifier 724, and a cable compensation circuit 750comprising resistors 753 and 755 and capacitors 752 and 754. Feedbackamplifier 724 includes programmable amplifier 725 and a feedback andvoltage divider circuit comprising resistors 726 and 728.

IC chip 202 includes 54 inputs 230, though for simplicity only nine areshown. Each input is connected to a 1/20 input divider, such as 762, andeach input divider is connected to one input of "A" muxamp 704 and oneinput of "B" muxamp 706. The output 770 of muxamp 704 is connected tothe input of programmable output stage 705, and the output 772 of theprogrammable output stage provides the "A" channel output of the chip. Adata signal is provided to the first 18:1 multiplexer 720 on line 780from the programmer 121, if this is the first chip, such as 202, in aprobe head, or from the last latch (not shown) in the preceding chip ifthis is not the first chip in the probe head. The data feeds frommultiplexer 720 via line 781 to the next multiplexer 721, then feeds tothe next multiplexer in the "A" channel via line 782, then to themultiplexers in the "B" channel on line 783, thence to the "B" channeloutput stage 707 via line 786, and then to the "A" channel output stagevia line 787. This IC chip is described more fully in U.S. patentapplication Ser. No. 08/369,607, now U.S. Pat. No. 5,629,617, which isincorporated herein by reference.

FIG. 6 shows a detailed circuit diagram of the input divider 762. Animportant feature of this divider is that it is on the IC chip 202,which allows one to take advantage of IC geometries that allow largenumbers of high impedance networks in small areas and still get lowcoupling. Input divider 762 includes input 230, GndF ground line 612,GndS ground line 614, compensation capacitor 602, and resistors 604,606, 608, and 610. The GndS ground is the "sense" ground or conventionalground of the bonding pad on which the IC is located, and the GndFground is a special current return ground to remove high frequencycurrent from the pad. Other capacitances that are not actual deviceswithin the divider network, but which must be considered so that thedivider functions as desired, are shown in FIG. 6. These include the padcapacitance 630, which is the net output capacitance of the bonding padon which the chip is located, amp capacitance 640, which is the inputcapacitance of the muxamp to which the output 763 of the input divider202 connects, stray capacitance 651 which is the stray capacitancebetween the input 230 and the GNDF ground 612, and stray capacitance652, which is the stray capacitance between the input 230 and the GndSground 614. The lines to the latter two capacitances are dotted toindicate that these are parasitic capacitances.

The input voltage divider network comprises resistors 604, 606, and 608in series between the input 230 and the output 763 of the input divider762, plus resistor 610 connected between output 763 and GndS ground 614.Capacitor 602 is connected in parallel with resistors 604 and 606between the input 230 and the node 603. This capacitor compensates forthe stray capacitances 651 and 652 and the amp capacitance 640. The padcapacitance occurs between the input 230 and the GndF ground 612, thestray capacitance 651 is indicated between the node 601 and the GndFground 612, and the stray capacitance 652 is indicated between the node603 and the GndS ground 614. The amp capacitance occurs between theoutput 763 and the GndS ground.

Preferably, capacitance 602 is 70 femtofarads, and resistors 604, 606,608 and 610 are 7.6 Kohms, 3.8 Kohms, 7.6 Kohms, and 1 Kohm,respectively. In the preferred embodiment, pad capacitance 630 is 125femtofarads, stray capacitance 651 is 20 femtofarads, stray capacitance652 is 10 femtofarads, and amp capacitance 640 is 120 femtofarads.

The total input impedance of input divider circuit 762 is the sum of thefour resistors 604, 606, 608, and 610 which totals 20 Kohms. A highinput impedance is important in an analog probe, since it prevents theprobe circuit from interacting with the circuit under test. However,making the input divider ratio too high, to attain higher inputimpedance, will attenuate the input signal to the point that the signalis too small to maintain good signal integrity. Moreover, when so manyinputs are in such a small area, i.e. when the input is very dense, itis difficult to provide high input impedance with low coupling betweenthe channels. It is an important feature of the invention that itcombines a high input impedance for each of more than a hundred inputswith high signal integrity.

There has been described a novel analog signal test probe which providesthe capability of selection of hundreds of probe channels and at thesame time provides high signal integrity and band width, which can beeasily and inexpensively customized for use with a particular circuitunder test, and which has many other advantages. It is evident that, nowthat the invention has been fully disclosed, those skilled in the artmay now make numerous uses and modifications of the specific embodimentdescribed, without departing from the inventive concepts. For example,now that it is seen that a ground personality board with fusibleconnections to ground provides ease of customization of a probe to aparticular circuit under test, others can now utilize the teachings todesign and manufacture many different varieties of analog probes. Orequivalent components or circuits can be substituted for the variouscomponents and circuits described. Additional features may be added. Agreater or lesser number of parts may be used. Consequently, theinvention is to be construed as embracing each and every novel featureand novel combination of features present in and/or possessed by thetest probe described.

What is claimed is:
 1. An analog voltage probe comprising:an analogamplifier; a plurality of terminals, each of said terminals adapted tobe connected to a pin of a circuit under test; a plurality of probeinput leads, each of said leads connected to one of said terminals andconnectable to said analog amplifier; a probe ground; and a fusibleelement connecting each of said probe input leads to said probe ground.2. An analog voltage probe as in claim 1 wherein said fusible elementcomprises an integrated circuit bond wire.
 3. An analog voltage probe asin claim 2 wherein said bond wire fuses at between 0.5 amperes and 1ampere current.
 4. An analog voltage probe as in claim 2 wherein saidbond wire comprises a metal selected from the group consisting of gold,aluminum, and platinum.
 5. An analog voltage probe as in claim 1 whereinsaid analog amplifier is on a probe circuit board and said fusibleelements are on a ground personality circuit board separate from saidprobe circuit board.